Fibromyalgia is a syndrome characterized by chronic and intense generalized pain, or widespread chronic pain over portions of the body; the pain is not limited to muscle tissue and may also be experienced in the skin. In fibromyalgia, such generalized chronic pain is often accompanied by symptoms including fatigue, malaise, depression, anxiety, muscle tightness in the morning, muscle stiffness and sleep disorders. Other accompanying symptoms also include headaches, facial pain, cognitive impairment (memory lapses, loss of concentration), gastrointestinal complaints (visceral pain, digestive system disorders, flatulency), frequent urination, diarrhea, constipation and dysmenorrhea.
It has been reported that 3.4% of women and 0.5% of men in the U.S. general population suffer from fibromyalgia. Moreover, fibromyalgia occurs more often in women generally between 25 and 50 years of age, with women accounting for approximately 80% of all patients. The prevalence rate in Japan are believed to be almost identical to the U.S. Although fibromyalgia has diverse subjective symptoms, objective findings have found few symptoms aside from a characteristic generalized tenderness. Even various immunological, virological and endocrinologic examinations as well as pathological examinations of myalgic regions, in addition to imaging examinations such as MRI and CT, have found almost no abnormalities. For example, although edemas do not occur as with rheumatoid arthritis and a blood index indicating a degree of inflammation, i.e., sedimentation or CRP, is within a normal range, patients complain of widespread pain covering the extremities and trunk.
Classification criteria put forward by the American College of Rheumatology in 1990 are currently used worldwide to diagnosis fibromyalgia. A diagnosis of fibromyalgia is made according to the following criteria: if pain must have been present for at least three months in all five regions of above the waist, below the waist, in the left side of the body, in the right side of the body, and also in the spinal and sternal region; or if pain must have been present in 11 or more sites of the 18 specified tender points when the palpation should be performed with an gentle force of 4 kg.
At present, the causes and mechanisms triggering the onset of fibromyalgia are not known, but are believed to include psychological factors brought on by stress or the like, viral infections, heredity, and immune and neural transmitter disorders. Fibromyalgia is a condition vastly different from many general painful conditions brought on by nociceptive stimulus, which damages or may possibly damage tissue, and no related pathological findings are observed on the pain regions.
Most anti-inflammatory analgesics such as non-steriodal anti-inflammatory drugs (NSAIDs), which are widely used for treating pain in general, are not very effective as treatment for fibromyalgia. Furthermore, various drugs including muscle relaxants, opioid analgesics and antianxiety agents have undergone trial use, but drug efficacy differs greatly among individuals and thus no prominent effect has been recognized. Consequently, as a current treatment of fibromyalgia, the drug therapy with antidepressants, the combined administration of antidepressants and NSAIDs, the administration of local anesthetics or steroids to painful sites, massages, therapeutic exercise, sleep therapy and the like are merely performed. However, the curative effects of all the therapeutic agents and methods differ greatly among individuals and have not been established as methods of the treatment, partly due to the fact that the cause of fibromyalgia has yet to be determined.
As explained above, given that the causes and mechanisms triggering the onset of fibromyalgia are not clear at present, and no drug has been found to demonstrate a prominent curative effect, medical facilities are in great need of a highly safe and effective therapeutic agent.
Thus, it is an object of the present invention to provide a highly safe and effective therapeutic agent for treatment of fibromyalgia, for which there is no efficacious medicine at present.